The vast majority of the world’s Muslim population follows the Sunni branch of Islam, while approximately 10-15% of all Muslims follow the Shiite (also Shi’ite, Shi’a, or Shia) branch. The differences between the Sunni and Shiite sects are rooted in disagreements over the succession to the Prophet Muhammad, who died in 632 AD, and over the nature of the leadership the Muslim community should have. The historic debate centered on whether to award leadership to a qualified, pious individual who would follow the customs of the Prophet, or to transmit leadership exclusively through the Prophet’s bloodline. The question was settled initially when community leaders elected Abu Bakr, a companion of the Prophet, to become the first Caliph (Arabic for “successor”). Although most Muslims accepted this decision, some supported the candidacy of Ali ibn Abi Talib, the Prophet’s cousin and son-in-law, husband of the Prophet’s daughter Fatima. Ali had played a prominent role during the Prophet’s life, but he lacked seniority within the Arabian tribal system and was bypassed.

This situation was unacceptable to some of Ali’s followers, who considered Abu Bakr and the next two succeeding caliphs (Umar and Uthman) to be illegitimate. Ali’s followers believed that the Prophet Muhammad himself had named Ali as successor and that the status quo was a violation of divine order. A few of Ali’s partisans orchestrated the murder of the third Caliph Uthman in 656 AD, and then Ali was named Caliph. Ali, in turn, was assassinated in 661 AD, and his son Hussein (680 AD) died in battle against forces of the Sunni caliph. Ali’s eldest son Hassan (d. 670 AD) is also revered by Shiite Muslims, some of whom claim he was poisoned by the Sunni caliph Muawiyah.

Those who supported Ali’s ascendancy became later known as “Shi’a,” a word stemming from the term “shi’at Ali,” meaning “supporters” or “helpers of Ali.” Others respected and accepted the legitimacy of his caliphate but opposed political succession based on bloodline to the Prophet. This group, who constituted the majority of Muslims, came to be known in time as “Sunni,” meaning “followers of [the Prophet’s] customs [sunna].”

Core Beliefs and Shared Principles

Although there are considerable differences between Sunni and Shiite Islam, the two Islamic sects share common traditions, beliefs, and doctrines. All Muslims believe that the Prophet Muhammad was the messenger of Allah. All believe that they must abide by the revelations given to the Prophet by Allah (as recorded in the Quran) and by the hadith (sayings of the Prophet and his companions). The concepts of piety, striving for goodness, and social justice are fundamental to Islamic belief and practice. Additionally, all Muslims are expected to live in accordance with the five pillars of Islam: (1) shahada—recital of the creed “There is no God but Allah, and Muhammad is His Prophet”; (2) salat—five obligatory prayers each day; (3) zakat—giving alms to the poor; (4) sawm—fasting from sunrise to sunset during the month of Ramadan; and (5) hajj—making a pilgrimage to Mecca once during a lifetime if one is physically and financially able.

Sunni Islam: Development and Basic Tenets

Sunni Muslims accept the first four Caliphs (including Ali) as the “rightly guided” rulers who followed the Prophet. In theory, Sunnis believe that the leader (imam) of the Muslim community should be selected on the basis of communal consensus, on the existing political order, and on a leader’s individual merits. This premise has been inconsistently practiced within the Sunni Muslim community throughout history. Sunni Muslims do not bestow upon human beings the exalted status given only to prophets in the Quran, in contrast to the Shiite veneration of imams. Sunnis have a less elaborate and arguably less powerful religious hierarchy than Shiites. Sunni Islam tends to be more flexible in allowing lay persons to serve as prayer leaders and preachers.

Within Sunni Islam, there are four schools of jurisprudence that offer alternative interpretations of legal decisions affecting the lives of Muslims. These four schools, which give different weight to the sayings of the Prophet and his companions (hadith) within their decisions, are:

Hanafi: This school was founded in Iraq by Abu Hanifa (d. 767 AD). It is prevalent in Turkey, Central Asia, the Balkans, Iraq, Syria, Lebanon, Jordan, Afghanistan, Pakistan, India, and Bangladesh.

Maliki: This was founded in the Arabian Peninsula by Malik ibn Anas (d. 795 AD). It is prevalent in North Africa, Mauritania, Kuwait, and Bahrain.

Shaf’i: This was founded by Muhammad ibn Idris al-Shafi’i (d. 819 AD). It is prevalent in Egypt, Sudan, Ethiopia, Somalia, parts of Yemen, Indonesia, and Malaysia.

Hanbali: This was founded by Ahmad Hanbal (d. 855). It is prevalent in Saudi Arabia, Qatar, parts of Oman, and the United Arab Emirates.

The Sunni puritanical movement called “Wahhabism” has become well known in recent years and is arguably the most pervasive revivalist movement in the Islamic world. This movement, founded in Arabia by the scholar Muhammad ibn Abd al-Wahhab (1703-1791 AD), is considered to be an offshoot of the Hanbali school of law. Abd al-Wahhab encouraged a return to the orthodox practice of the “fundamentals” of Islam, as embodied in the Quran and in the life of the Prophet Muhammad. In the eighteenth century, Muhammad ibn Saud, founder of the modern-day Saudi dynasty, formed an alliance with Abd al-Wahhab and unified the disparate tribes in the Arabian Peninsula. Since that time, there has been a close relationship between the Saudi ruling family and the Wahhabi religious establishment. The most conservative interpretations of Wahhabi Islam view Shiites and other non-Wahhabi Muslims as dissident heretics. Following the 1979 Soviet invasion of Afghanistan and the Shiite Islamic revolution in Iran, Saudi Arabia’s ruling Sunni royal family began more actively promoting Wahhabi religious doctrine abroad. Saudi individuals and organizations since have financed the construction of Wahhabi-oriented mosques, religious schools, and Islamic centers in dozens of countries.

Shiite Islam: Development and Basic Tenets

For Shiites, the first true leader of the Muslim community was Ali, who was considered an imam, a term used among Shiites not only to indicate leadership abilities but also to signify blood relations to the Prophet Muhammad. As Ali’s descendants took over leadership of the Shiite community, the functions of an imam became more clearly defined. Each imam chose a successor and, according to Shiite beliefs, he passed down a type of spiritual knowledge to the next leader. Imams served as both spiritual and political leaders. But as Shiites increasingly lost their political battles with Sunni Muslim rulers, imams focused on developing a spirituality that would serve as the core of Shiite religious practices and beliefs. Shiites believe that when the line of imams descended from Ali ended, religious leaders, known as mujtahids, gained the right to interpret religious, mystical, and legal knowledge to the broader community. The most learned among these teachers are known as ayatollahs (lit. the “sign of God”).

Shiite religious practice centers around the remembrance of Ali’s younger son, Hussein, who was martyred near the town of Karbala in Iraq by Sunni forces in 680. His death is commemorated each year on the tenth day of the Islamic month of Muharram in a somber and sometimes violent ritualistic remembrance known as “Ashura,” marked among some Shiites by the ritual of self-flagellation. As a minority that was often persecuted by Sunnis, Shiites found solace in the Ashura ritual, the telling of the martyrdom of Hussein and the moral lessons to be learned from it.

"Twelver Shiism"—the most common form of Shiism today—is pervasive in Iran, Iraq, Lebanon, and Bahrain. Twelvers accept a line of twelve infallible imams descendent from Ali and believe them to have been divinely appointed from birth. The twelve imams are viewed as harbors of the faith and as the designated interpreters of law and theology. Twelvers believe that the twelfth and last of these imams “disappeared” in the late ninth century. This “hidden imam” is expected to return to lead the community.

Although most Shiites agree on the basic premise that Ali was the first rightful imam, they disagree on his successors. The Ismailis, who are the second largest Shiite sect, broke off in the eighth century, recognizing only the first seven imams (the seventh was named Ismail, hence the names “Ismaili” and “Sevener”). Historically and at least until the sixteenth century, the Ismailis were far more disposed than the Twelvers to pursuing military and territorial power. In the past, they established powerful ruling states, which played significant roles in the development of Islamic history. Today, Ismailis are scattered throughout the world but are prominent in Afghanistan, India, and Pakistan. There are also Ismaili communities in East and South Africa.